Is a Solid Support only a Protective Group?

R C X H Drugs are polyfunctional molecules to provide the highest density of chemical functionalities with well-defined orientation in space per surface. X R Various functional groups H Hydrophobic residues C - electron clouds Polar residues

How to deal with polyfunctional compounds? deprotection protection work up purification

Advantage of polymeric support immobilization cleavage wash wash wash evaporation (purification) Rapid procedure The physical properties of the polymer differ from those of small molecules facilitating isolation

Schematic representation of resins swollen resin (solvated) shrunken resin (poorly solvated) cross link compound bound to the polymer Reactions can only be performed, if reagents can diffuse through the gel phase of the beads. The polymer has to be solvated with the appropriate solvent to form such a phase.

Resin: Amorphous polymer Polymeric support: Chemically inert carrier of linker, spacer and scaffold. Spacer: provides distance between support and linker rotational and vibrational freedom of scaffold provides higher (solution-like) reactivity of scaffold Linker: allows selective cleavage of scaffold from polymeric support

Fixing nucleophiles on “Rink resin” Isolated yields of recovered TFA-salts after cleavage with 20% TFA in DCM.

Fixing piperazin on “Rink resin” Brill, W. K.-D.; Schmidt, E.; Tommasi, R. A. Synlett (1998), (8), 906-908

The principle of “infinite dilution” The regidity of the resin prevents double attack

Immobilization of purines

C6-Substitution A B C

Reactions of linker under too acidic conditions Basic

C6-Substitution 53°C to avoid substitution on C2 for very nucleophilic amines (N-CH2CH2NH2, C-CH2CH2NH2 piperazines, piperidines). No C2-substitution even at 70°C: anilines, benzylamines, morpholin, primary amines with higher order of substitution on their Ca

Reaction of two neighboring molecules

Interactions of purines with each other on a solid support Brill : 03.1999 Interactions of purines with each other on a solid support Batch 1 is hydrolysable, batch 2 is not. Batch 1: Batch 2:

Heat-shock disrupts rigid resin structure and allows further reaction Brill : 03.1999 Heat-shock disrupts rigid resin structure and allows further reaction Batch 2:

Schematic representation of resins swollen resin (partially loaded) Non-covalent crosslinks Non-covalent crosslinks prevent further loading of resin More rapidly diffusing reagents might cause side reactions usually not observed in solution phase

Reaction Scheme

Reaction Scheme

Two types of bromine complexes were investigated Synthesis: Reactivity:

Where is the resin being modified? Can the bromine be eliminated?

Modification of the polymeric support by the brominating agent Bromine-complex Br-content of resin, treated with the Bromine Complex (EA)

The resin was modified on the side chains The bromine can be eliminated. NMP prevents modification of the resin The linker may be modified, but the modification is tolerable – linker properties are not changed a lot.

Reactivity of the bromine complex with purines: Solvent: NMP no reaction: 5% conversion complete conversion: R1: H, alkyl, R1’: alkyl R2: H, alkyl, R2’: H, alkyl tolerated groups: CONR2, CONRH CONH2, ether functions, aromatics more deactivated than Bn side reactions: not tolerated groups: activated aromatics: bromination amines: many oxidation products

The reactivity of the bromine complex in dry benzene: disproportionation is very slow little electrophilic substitution oxidizing power is enhanced relative to reactions in NMP Selective side reactions:

The reactivity of the bromine complex in wet benzene: Side chain oxidations are not mediated by the solid phase. They work are observed also in solution phase.

Proposed mechanism for the modification at C2 i: Br2-complex; ii: H2O Riva-Toniolo, C.; Mueller, S.; Schaub, J.; Brill, W. K.-D. Molecular Diversity (2003), 6(1), 43-53

Diversification of cinnolines on solid phase

Diversification of cinnolines on solid phase

Acylation of SP by cinnolines 4-chlorobenzylamine aromat. C-H out of plane strech NH 4-chlorobenzylamide NO- strech aromat. C-H out of plane incomplete complete

Analysis of resins by MAS-1H-NMR High Resolution MAS- 1H-NMR Analysis of resins by MAS-1H-NMR Homonuclear Dipolar Couplings & Magnetic Susceptibility-induced Linebroadenings removed by 1-3 kHz MAS* and Magnetic Susceptibility Matching Probe Technology (Nano Probe) * MAS = 54°44’ (3cos2 -1 = 0) Gradients & Inverse Detection Capability NanoProbe 40l Cell

1H-MAS-NMR analysis of acylation of SP by cinnolines

Cinnolines become trapped in resin before reacting

Alkylation Sereni, L.; Tato, M.; Sola, F.; Brill, W, K.-D. Tetrahedron (2004), 60(38), 8561-8577

Synthesis of oxadiazoles on solid phase

The LP-TFAA mediated cyclization gives three products B C The SP-TFAA mediated cyclization gives two products, but never B Only R1 is released as acid, never R2

SP and LP cyclization give different products and display different solvent dependence B C A B : C Elemental analysis of resin after cyclization: In a SP cyclization with 50% yield, 50% of the initial N remains on the resin R1CO2H (A) is not released in SOCl2-mediated cyclizations

Not observed in THF The LP-TFAA mediated cyclization-mechanism in THF Rate limiting Not observed in THF

How deep can one view into the resin? MAS-1H-NMR Reagents in solution Normal 1H-NMR

The SP- TFAA mediated cyclization gives two products

The SP-TFAA mediated cyclizations proceed via different intermediates

The SP- TFAA mediated cyclization gives two products Irreversible sequestration H+ A C

recovery of all products Isolated (LP-model) Sequestration: recovery of all products Detected from LP-model A isolated Calc.: C30H26N2O5 + H+ 495.1914 Found: 495.1925 C isolated

Attack of TFAA on SP-bound hydrazide-conformers

The reaction path is determined by which carbonyl was initially attacked

Effect of SP Crude product from solid phase Product from solid phase after treatment with basic cation exchanger Crude product from liquid phase

Conclusion Conclusion Conclusion Conclusion Conclusion Conclusion Resins can work as protective groups. (immobilization of amines) Resins may sometimes provide the effect of infinite dilution. (monoacylation of piperazine) Resin attachments may migrate. (purines) Resin-bound substrates may interact to give covalent crosslinks violating the effect of infinite dilution. (purines) Resin-bound substrates may interact to give non-covalent crosslinks leading to rigid structures and preventing desired reactions. (purines, cinnolines) Resins enable reactions not observed in solution phase. (oxadiazoles) Resins may be modified during reactions, but not all modifications cause problems. (bromination) Conclusion Resins can work as protective groups. (immobilization of amines) Resins may sometimes provide the effect of infinite dilution. (monoacylation of piperazine) Resin attachments may migrate. (purines) Resin-bound substrates may interact to give covalent crosslinks violating the effect of infinite dilution. (purines) Resin-bound substrates may interact to give non-covalent crosslinks leading to rigid structures and preventing desired reactions. (purines, cinnolines) Resins enable reactions not observed in solution phase. (oxadiazoles) Resins may be modified during reactions, but not all modifications cause problems. (bromination) Analysis is difficult, but only needed in case of problems. Conclusion Conclusion Resins can work as protective groups. (immobilization of amines) Resins may sometimes provide the effect of infinite dilution. (monoacylation of piperazine) Resin attachments may migrate. (purines) Resin-bound substrates may interact to give covalent crosslinks violating the effect of infinite dilution. (purines) Resin-bound substrates may interact to give non-covalent crosslinks leading to rigid structures and preventing desired reactions. (purines, cinnolines) Resins enable reactions not observed in solution phase. (oxadiazoles) Conclusion Resins can work as protective groups. (immobilization of amines) Resins may sometimes provide the effect of infinite dilution. (monoacylation of piperazine) Resin attachments may migrate. (purines) Resin-bound substrates may interact to give covalent crosslinks violating the effect of infinite dilution. (purines) Resin-bound substrates may interact to give non-covalent crosslinks leading to rigid structures and preventing desired reactions. (purines, cinnolines) Conclusion Resins can work as protective groups. (immobilization of amines) Resins may sometimes provide the effect of infinite dilution. (monoacylation of piperazine) Conclusion Resins can work as protective groups. (immobilization of amines) Resins may sometimes provide the effect of infinite dilution. (monoacylation of piperazine) Resin attachments may migrate. (purines) Conclusion Resins can work as protective groups. (immobilization of amines) Resins may sometimes provide the effect of infinite dilution. (monoacylation of piperazine) Resin attachments may migrate. (purines) Resin-bound substrates may interact to give covalent crosslinks violating the effect of infinite dilution. (purines) Conclusion Resins can work as protective groups. (immobilization of amines)

Conclusion Resins can work as protective groups. (immobilization of amines) Resins may sometimes provide the effect of infinite dilution. (monoacylation of piperazine) Resin attachments may migrate. (purines) Resin-bound substrates may interact to give covalent crosslinks violating the effect of infinite dilution. (purines) Resin-bound substrates may interact to give non-covalent crosslinks leading to rigid structures and preventing desired reactions. (purines, cinnolines) Resins enable reactions not observed in solution phase. (oxadiazoles) Resins may be modified during reactions, but not all modifications cause problems. (bromination) Analysis is difficult, but only needed in case of problems.

Eduard Felder Ruben Tommasi Erika Schmidt Claudia Riva Josette Schaub Sascha Mueller Sara Cesarini Maurizio Pulici Laura Sereni Francesco Sola Massimo Bartolacci Antonio Traversone Marco Tatò